Many wind turbines use either a high ratio gearbox (e.g. 80:1) or none at all (1:1). The challenge for using no gearbox at all is the very low speed of the main axle or input shaft. Thus, to get a usable induction and production of electricity the generator needs to have a huge area of active magnets, and as many poles as possible together with many windings. In practice, the size of the generator is enormous.
On the other hand using a high ratio gearbox brings up the speed and minimizes the generator size. However, the cost of a high speed gearbox is loss of power during conversion, which accumulates to almost 1% for each stage. Using a three stage planetary gearbox means loss of 3% power before entering the generator. This lowers the profit of utilizing the turbine and emphasizes the need for cooling the gearbox properly during operation. Just as an example, having a generator with an efficiency of 97% and in the same drivetrain a gearbox of 97% gives a conversion efficiency of 94% which means 6% is lost in heat inside the transmission. Converting 3 MW means that 180 kW is lost as heat, and must be removed from the transmission to the ambience. This may be a challenge apart from representing a loss in economy.
In a direct drive solution 97% of efficiency may be achieved in the generator as well, and will leave out the losses of the gearbox transmission.
Many manufactures of wind turbines still use gearboxes in new designs, as this lowers size and weight of the generator. However, the gearbox may—apart from losing power during conversion—also be a challenge in other ways.
The rotor of the wind mill produces an extremely high torque during operation. Producing e.g. 3 MW with only 15 rpm rotor speed means that the torque entering a gearbox in the wind turbine reaches nearly 2 MNm, which is almost equivalent to the torque of 10,000 automotive engines put together. Thus, there is a need for rapid distribution or conversion to higher speed with lower torque in order to keep the wind turbine reasonable in size and weight.
Today both distribution of power and conversion to higher speed are ways seen in practice.
In WO2012156128 a widely used gearbox is shown, in which the torque is transformed using a single stage planetary gear, followed by two simple helical gear stages.
Using a planetary stage as input gear gives a natural distribution of the torque from rotor to several planetary wheels, that together help to convert the low speed input to a high speed output, which may be handled by the following helical gear stages.
Using a planetary gearbox is complicated even though it is a good means to distribute the high torque from the rotor. Planetary gears may be useful for securing that all planetary wheels carry the same load while avoiding overstressing one or more. In practice this calls for a high precision in the position and production of gearwheels and planetary carrier, as well as the housing of the gearbox. Having such restrictions in design and production increases the cost of the gearbox.
For the gearbox having a first planetary stage, the connection towards the rotor is very critical. In many gearboxes using planetary gears as a first stage, the input of the gear stage is the planet carrier. This is usually fixed to the housing of the gearbox rotationally using one or more bearings. Thus the connection towards e.g. a main axle transferring the power from the rotor of the wind turbine to the gearbox is very critical. The main axle is very typically fixed rotationally by one or two bearings, meaning that a stiff or non-flexible connection between the main axle and the gearbox results in over constraints. In the end any flexing of the main axle during operation e.g. during high winds produce unexpected loads of the planet carrier, and possible bendings, leading to unpredictable loads of the individual gearwheels. Consequently, the gearbox loose lifetime of operation or even breaks down.
Finally, the usage of multiple gearwheels in a planet stage may during certain conditions lead to oscillation between the gearwheels, depending on many factors such as stiffness and loads/speeds.
Clipper Wind (WO2012164501) suggest to split out the high torque from the rotor using 4 individual bull gears placed on the main shaft, each meshing with a pinion driving a second stage gearbox. Finally the second stage gearbox is connected to a generator converting the energy. As opposite of the planet gearbox the concept offers load distribution to four separate sub-systems early in the drivetrain. Depending on the control of each generator, the concept offers independent power control, which is expected to give lower interference and by that phenomenon such as oscillations and vibrations. At least such phenomenon may easily be controlled using the loading of generators to eliminate it.
In addition the concept offers much redundancy as the system is expected to be operational even though one of four systems sets out for one or another reason.
In many dimensions, the later example gives a high reliability and robustness. In addition, it gives a reasonable compact size, and freedom to adapt one or more embodiments for different input powers.